As usual, way to late for this, but for (halfway) completeness ... (Note: This info comes with "froob" as context; players with just SL or even all expansions have a number of good choices outlined in the soldier guide open to them).
I see a good number of players replace (or trying to replace) their Neutrino Flash with a Hellspinner. It usually doesn't work out as good as expected, because compared to the Neutrino Flash the Hellspinner is a "boring" weapon (small clip, long recharges on specials).
Div9 is indeed a decent replacement, but should be used, if possible, with a TIM scope
(see Biomare guide
). Personally I went with a Nova Flow MkIV (not upgraded), which is faster and thus offers the option of being used with a crit scope.
Similar to the Nova Flow MkIV, but without the Ranged Energy skill req, comes the MTI Cineration series of guns.
Other alternatives are the already mentioned BBI Gyro, Eradicator XCI-52 or the turnspirit
assault rifles for omni/clan players: Spastic/Cooperator Assault Rifle, respective.
There is also the option to jump off from the Assault Rifle train and for example go with Blackbird guns.
Nova Flow MkIV and Eradicator XCI-52 can be rolled as mission reward.
Division 9, BBI Gyro and the MTI Cineration (latter only in available the range of QL165-175) are dynaboss drops, and Div9 + BBI Gyro drop also from mission bosses iirc.
Blackbird is dropped from the Cenobite Shadow mobs in CoH, Crypt of Home
Don't use the normal Hellfury, it's much worse than the Neutrino Flash. Both Hellfury and Hellspinner can be upgraded, as said already, but the first upgrade (Boosted ~) increases the level lock from 71 to 175. The Nophex II gun follows step in the Nova Flow MkIV series and is dropped by the Notum Soldier
(might be easier to let someone pick it up from GMS).